build-sys: make SOURCE_XML_FILES array relative to $(top_srcdir)

build-sys: make SOURCE_XML_FILES array relative to $(top_srcdir)

When calling the build commands from another directory than the toplevel:

  mkdir build
  cd build
  ../configure [...]

... the compilation fails with the following error:

 GEN      man/systemd.directives.xml
Traceback (most recent call last):
  File "../tools/make-directive-index.py", line 313, in make_page
    _extract_directives(directive_groups, formatting, page)
  File "../tools/make-directive-index.py", line 191, in _extract_directives
    t = xml_parse(page)
  File "/home/martin/upstream/systemd/tools/xml_helper.py", line 30, in xml_parse
    doc = tree.parse(page, _parser)
  File "lxml.etree.pyx", line 3301, in lxml.etree.parse (src/lxml/lxml.etree.c:72453)
  File "parser.pxi", line 1791, in lxml.etree._parseDocument (src/lxml/lxml.etree.c:105915)
  File "parser.pxi", line 1817, in lxml.etree._parseDocumentFromURL (src/lxml/lxml.etree.c:106214)
  File "parser.pxi", line 1721, in lxml.etree._parseDocFromFile (src/lxml/lxml.etree.c:105213)
  File "parser.pxi", line 1122, in lxml.etree._BaseParser._parseDocFromFile (src/lxml/lxml.etree.c:100163)
  File "parser.pxi", line 580, in lxml.etree._ParserContext._handleParseResultDoc (src/lxml/lxml.etree.c:94286)
  File "parser.pxi", line 690, in lxml.etree._handleParseResult (src/lxml/lxml.etree.c:95722)
  File "parser.pxi", line 618, in lxml.etree._raiseParseError (src/lxml/lxml.etree.c:94754)
OSError: Error reading file 'man/bootup.xml': failed to load external entity "man/bootup.xml"

That is because the file names in the XML_FILES array are not relative
to $(top_srcdir), and hence ../tools/make-directive-index.py is called
with non-existant arguments.

To fix this, call patsubst when generating SOURCE_XML_FILES from
NON_INDEX_XML_FILES.